1. Field of the Invention
The present invention relates to a linear image sensor adapted for use in an image reading apparatus such as a copying machine or a scanner.
2. Related Background Art
Conventionally, it is well known that linear image sensors are utilized in image reading apparatus, and CCD linear image sensor is a representative example of such linear image sensors.
FIGS. 16A and 16B are schematic views showing the configuration of such linear image sensor. Referring to FIG. 16A, there are shown photoelectric converting pixel elements 201-a, b, c, d, . . . arranged with a constant pitch in a one-dimensional array; shift gates 202-a, b, c, d, . . . for shifting the charges obtained by the photoelectric conversion in the photoelectric converting pixel elements 201-a, b, c, d, . . . ; a transfer unit 203 for successive transfer of thus shifted charges; and an output circuit 204 for releasing thus transferred charges in the form of sequential output signals. FIG. 16B is a magnified view of the photoelectric converting pixels 201-a, b in FIG. 16A.
In the following there will be explained the function of such CCD linear image sensor when it is employed in an image reading apparatus such as a scanner or a digital copying machine.
The original image to be read is illuminated in a linear shape, and an optical image focused on the array of the photoelectric converting pixels 201-a, b, c, d, . . . for example through an optical lens moves in the sub scanning direction shown in FIG. 16B (hereinafter represented as y-direction) at a predetermined speed. In a predetermined period required by the focused image to move from a position A to B shown in FIG. 16B, the charges accumulated by the photoelectric conversion in the photoelectric converting pixels 201-a, b, c, d, . . . are shifted to the transfer unit 203, and the corresponding output signals are released from the output circuit 204 in a predetermined period required by the focused image to move from the position B to C.
In general, as shown in FIG. 16B, the distance AB (and distance BC) is selected equal to the pitch P of the photoelectric converting pixels in the main scanning direction (hereinafter represented as x-direction), in order to obtain a same resolving power in the x- and y-directions.
In such conventional configuration, in order, for example, to double the resolving power, it is necessary to reduce the pitch P of the photoelectric converting pixels to 1/2, by reducing the pixel size of the photoelectric converting pixels 201-a, b, c, d, . . . to 1/2 both in the x- and y-directions. It is also necessary to reduce the above-mentioned periods to 1/2.
In such case, the area of the photoelectric converting pixel is reduced to 1/4 and the photoelectric converting time is also reduced to 1/2, so that the sensitivity is reduced to 1/8, whereby the image quality is significantly deteriorated. Moreover the doubled transfer rate adversely affects the charge transfer performance, and also leads to an increase in the heat generation in the transfer unit and in the power consumption, and these factors naturally affect disadvantageously the quality of the image obtained in the image forming apparatus.
Among the above-mentioned factors, the deficiency in sensitivity may be coped with by an increase in the size of the photoelectric converting pixels and a reduction in the imaging magnification of the lens. For example the original sensitivity may be recovered by increasing the size of the photoelectric converting pixels by about 2.8 times both in the x- and y-directions, but, in such case, the transfer unit also becomes larger in size, thus giving rise to an even larger deterioration of the charge transfer performance and an even large heat generation, whereby the deterioration of the image quality inevitably becomes more conspicuous. Also the chip size becomes about 2.8 times in the x-direction, leading to a major increase in the cost of the linear image sensor and the lens.